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7. Plus One

easyAsked at Yelp

Increment a big integer stored as a digit array — Yelp uses it to gauge how cleanly you handle carry-propagation, the same edge case in totaling review counts past 9.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

You are given a large integer represented as a non-empty array of digits where each element is a single digit. Add one to the integer and return the resulting digit array.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digits[i] <= 9
  • Leading digit is non-zero unless the number is 0

Examples

Example 1

Input
digits = [1,2,3]
Output
[1,2,4]

Example 2

Input
digits = [9,9]
Output
[1,0,0]

Approaches

1. BigInt convert

Join, convert to BigInt, add one, split.

Time
O(n)
Space
O(n)
const n = BigInt(digits.join('')) + 1n;
return n.toString().split('').map(Number);

Tradeoff:

2. Right-to-left carry

Sweep from the back; if digit was 9 set to 0 and continue, else increment and return.

Time
O(n)
Space
O(1)
function plusOne(digits) {
  for (let i = digits.length - 1; i >= 0; i--) {
    if (digits[i] < 9) { digits[i]++; return digits; }
    digits[i] = 0;
  }
  return [1, ...digits];
}

Tradeoff:

Yelp-specific tips

Yelp wants the carry-propagation solution that avoids BigInt and will ask how the same pattern increments aggregate review counters with overflow protection.

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Output

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